This invention relates to the integrated-circuit fabrication of high performance logic operators derived from logic inverters which make use of Schottky-gate field-effect transistors (MESFET's) having a low threshold voltage, especially operators which achieve up to three logic levels per gate and have a high entrance.
It is known that, in a complex logic gate, there can be distinguished a number of logic levels or layers corresponding to the number of elementary operators on the path of the logic signals. When the elementary operators are active components such as diodes and transistors, the digital signal experiences a time-delay as it passes through each operator. In order to improve the speed of the logic circuit, it is therefore an advantage to reduce the number of logic layers. Furthermore, the aim of the invention is to improve the entrance or in other words to permit simultaneous control of the operator by means of a large number of inputs without any impairment of operational safety, even in the presence of noise.
Field-effect transistors of the so-called "normally-on" or "normally-off" type are already known. Normally-on transistors operate in the depletion mode, which calls for double-polarity supply and also has the disadvantage of consuming power in the quiescent state. Normally-off transistors permit the passage of current when a suitable voltage is applied to the control gate; thus they do not require a double-polarity supply and are more economical. On the other hand, they are difficult to construct by reason of the very small thickness of the conduction channel and give rise to a large number of production rejects.
A third class of field-effect transistors which is in an intermediate category between the two classes mentioned above while being more closely related to the second class consists of quasi-normally-off transistors. These transistors have a threshold voltage V.sub.T which can be either positive or negative; this means that a certain degree of dispersion is tolerated in the collective manufacture of these transistors and is, for example, as follows: EQU -0.4.ltoreq.V.sub.T .ltoreq.0.2 volt.
The third class of transistor is easier to fabricate than the second class but more difficult to use in practice. It is known to construct transistors having a low threshold voltage wherein a trench is cut by ionic erosion between the source and the drain, thus constituting a saturable resistor having very low power consumption which can be converted to a field-effect transistor by depositing a metallized coating at the bottom of the trench at the control gate location so as to perform the function of this latter. However, transistors fabricated by means of this technology exhibit a certain degree of dispersion and are therefore subject to certain disadvantages by reason of the dispersion of characteristics. Thus the threshold voltage is sometimes positive and sometimes negative; in the event that the voltage is negative, the input transistor is not pinched-off, thereby producing a reduction of entrance and making it impossible in practice to form multiple inputs.